In the past, there has been great interest in determining the load carried on trucks for hire. Loads carried by trucks are often supported by pallets because pallets provide a stable platform on which to position and transport goods between trucks, using a hydraulic lifting device. Typically, prior art weight sensing devices are concerned with measuring the change in the gross weight of a truck to determine whether it complies with regulations relating to the loading of commercial motor vehicles and/or determining the amount to charge a customer for transportation of a particular load. This does not allow determining the weight of individual pallets of goods before loading the pallets onto the truck, which may result in overloading of the same.
To prevent overloading of the truck, the pallets of goods may be individually transported to a scale where they are then weighed. A problem with this procedure is that it is time consuming, resulting in increased transportation costs for a given load. An obvious solution to this problem would be to provide a scale for each truck receiving a load, thereby providing dynamic weighing of the total load of the truck as pallets of goods are placed thereon. This would require having a separate scale for each truck receiving pallets of goods, or creating a queue of trucks for each scale present so that each may in turn be placed on the scale during loading. Both of these solutions result in the same drawbacks as individually weighing pallets of goods.
Prior art attempts have been made for weighing of loads on forklifts. Weigh-Tronix, Inc. describes, in a sales brochure, a device for weighing loads supported by a forklift. The Weigh-Tronix device includes a large frame fitting between the carriage and a pair of forks. The frame is mounted parallel to the carriage and includes two plates parallel to the carriage and the forks. The first plate is affixed to the carriage of the forklift. The second plate is constructed similarly to the forklifts original carriage, and serves as the attachment point of the forks. Load cells are mounted between the plates. These load cells deflect as the load is placed on the forks, causing the two plates to deflect vertically with respect to each other, while remaining parallel. The drawback with this system is the construction of the two plates requires substantial material. This, coupled with the shift of the load center away from the forklift, results in a reduction in capacity in capacity for the forklift. In addition it is expensive to manufacture and transport to the end user. The load cell in this system are also subject to fatigue and may deform if overloaded, making the device susceptible to premature failure.
To overcome the excessive weight of the frame, prior art devices have placed a weight transducer in the hydraulic lift circuit. In this manner, a portion of the hydraulic fluid is transmitted along a bypass from the main lift circuit. A transducer is positioned to measure the pressure of the fluid in the bypass.
U.S. Pat. Nos. 5,287,885; 5,195,418; and 5,139,101 to Smith each discloses such a bypass system. Specifically, a motion control system for hydraulically operated lifting devices is shown including, in pertinent part, a two-way valve having a normally open valve in one chamber and a normally closed valve in another chamber. The normally open valve may be closed to re-direct flow of hydraulic fluid from a main valve, under pressure from a hydraulic pump, to a bypass chamber having a flow control valve. The normally closed valve may be opened to direct flow from a lift circuit of the second flow control valve to pass the hydraulic fluid back to a hydraulic fluid reservoir tank. These bypass systems are, however, slow to operate and may not be operated during the normal course of the forklift's operation. They are also susceptible to error from the chains of the lift, as will be described herein.
Load cells overcome the drawbacks of hydraulic bypass systems. U.S. Pat. No. 5,461,933 to Ives et al. discloses a compressive load cell having a shear web design. The load cell includes, in pertinent part, an annular body concentrically disposed about a longitudinal axis of a cylindrical body. Two pairs of flexible webs interconnect the annular body, with each web in a pair disposed opposite of the remaining web. The webs are spaced-apart about the cylindrical body, 90.degree. from an adjacent web. Two strain-gauges are attached to each web on opposing surfaces. The strain-gauges are connected in a wheatstone bridge configuration, which is provided with thermally-compensating potentiometers.
U.S. Pat. No. 4,212,360 to Chesher discloses a load weighing system for a forklift. The system includes, in pertinent part, load cell transducers disposed to measure compressive forces exerted between a supporting chain and the anchorage of the chain. The chain supports the carriage to which lifting forks of the forklift are attached. The load cell transducers are formed from a spool-shaped body that has a plurality of resistive strain-gauge elements affixed to it using a suitable adhesive. The strain-gauge elements are connected in a bridge arrangement such that the degree of unbalance of the bridge is a measure of the magnitude of the compressive forces applied across the ends of the spool-shaped body.
U.S. Pat. No. 2,813,709 to Brier discloses a strain-gauge load cell that includes strain-gauges attached to suspension springs of trucks. Also included is a compensation gauge. The compensation gauge and the strain-gauges are both electronically coupled to a bridge circuit. The total impedance across the bridge circuit is affected by both the load on the strain-gauges and the temperature sensed by the compensation gauge. The compensation gauge is chosen so that the impedance across the bridge circuit is constant for a particular weight upon the strain-gauge regardless of the temperature of the surrounding atmosphere.
U.S. Pat. No. 2,582,886 to Ruge discloses a differential load device that includes a load cell having a load button. A helical compression spring is disposed around the load cell. The spring is chosen to receive a predetermined base weight, with any additional load, or differential load, being distributed between the load cell and the spring. The spring includes two strain-gauges on the outer surface of the wire that forms the spring. Typically, the spring is chosen so that its axial stiffness is relatively small compared to that of the load cell. In this manner, the load cell is rendered extremely sensitive to any differential load that is placed on it. The two gauges allow detection of the total load on the device, i.e. the base load and the differential load.
Many of the aforementioned load cells are directed to accurate measurements of an applied load despite environmental anomalies. Such environmental anomalies include ambient temperature fluctuations. However, the accuracy of the load measured is often degraded due to various mechanical disturbances, e.g. shaking and vibration,. as well as the position of the load on the cell.
What is needed is a lifting device capable of producing a high precision weight measurement, dynamically, without degradation of the measurement's accuracy by mechanical disturbances in the lifting device.